Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further postexposure overall treatment. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate.
Traditionally, the plate is exposed with an ultraviolet light from a lamp through a separate photomask film having predetermined imaging pattern that is placed between the light source and the plate. Laser sources have been increasingly used to imagewise expose a printing plate that is sensitized to a corresponding laser wavelength, allowing the elimination of the photomask film. Suitable lasers include, for example, infrared lasers (such as laser diode of about 830 nm and NdYAG laser of about 1064 nm), visible lasers (such as frequency-doubled NdYAG laser of about 532 nm and violet laser diode of about 405 nm), and ultraviolet laser (such as ultraviolet laser diode of about 370 nm). Laser sensitive plates generally have higher sensitivity (than conventional plates) because of the limited power of current laser imagers.
Conventionally, the exposed plate is developed with a liquid developer to bare the substrate in the non-hardened or solubilized areas. On-press developable lithographic printing plates have been disclosed in recent years. Such plates can be directly mounted on press after imagewise exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed, allowing the reduction of labor and the elimination of hazardous waste. Among the patents describing on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, 6,071,675, and 6,482,571.
While laser sensitive on-press developable plates have the advantage of not requiring a photomask film and not requiring a separate wet development process, such plates often have the drawbacks of insufficient laser sensitivity, insufficient durability, insufficient resolution, limited room light stability and/or weaker visible images. Also, it is hard to design a plate with combined good on-press developability, good photospeed, good durability and good resolution, because, for example, the factors which favor on-press developability often offset the small dots resolution, photospeed or durability. Furthermore, it is particularly hard to design an overcoat-free laser sensitive on-press developable lithographic plate with good photospeed; and the requirement of overcoat for laser sensitive on-press developable lithographic plate has been a major barrier for many manufacturers to make laser sensitive on-press developable plate, because most small to medium plate manufacturers currently do not have 2-layer coating facility which is needed for making a plate with overcoat. It would be desirable if the above drawbacks or problems could be eliminated through a simple prepress process which does not involve the use of chemicals.
The inventor has found that a simple heating method can be used for an overcoat-free lithographic plate after laser exposure and before on-press development to enhance or enable the hardening or solubilization of the exposed areas of the photosensitive layer so as to improve the performance or to allow proper performance of the plate.